This invention relates to an apparatus for limiting the maximum number of stitches of sewing machines driveable by electrical regulating and control drives. The drive consists of a motor, a clutch and brake part associated with the motor, and a control part for the clutch and brake. At least one potentiometer and at least one switch are connected ahead of or beyond the latter to limit the maximum rotational speed of the drive.
Electrical regulating and control drives of this type for sewing machines are known from the journal Bekleidung und Wasche, No. 7, 1970, pp. 466 to 470. Here, the motor rotates continuously, while the sewing machine is connected to the motor alternately or simultaneously using the clutch and brake. The clutch and brake are actuated by the control part, for example as a function of the needle position, in order to ensure that the sewing machine stops in a preset, for example an upper, dead-point position of the needle. In addition, such a control part can also serve for actuating a thread cutter, thread layer, a device to reduce thread tension, and the like. The control part is conventional and can be constructed as shown in the U.S. Pat. No. 3,407,910 granted Oct. 29, 1968 to Heidt.
Industrial sewing machines equipped with such drives operate at up to 6000 stiches per minute. For example, when sewing curves or when sewing fancy seams with particularly long stitches, the sewing machine cannot be driven at the rate which produces the maximum number of stitches, because the thread would break in the case of especially long stitches or the seamstress could no longer guide the material rapidly enough along the predetermined curve when sewing on curves. In order to provide assistance in this situation, it is already known to limit the maximum number of stiches of a sewing machine by limiting the maximum rotational speed of the drive, providing the control part with a potentiometer associated therewith, the potentiometer having a switch connected ahead of or beyond it, the switch being in the form of a position switch, i.e., a toggle or rotary switch.